gecko-dev/js/public/TracingAPI.h

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 * vim: set ts=8 sts=4 et sw=4 tw=99:
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#ifndef js_TracingAPI_h
#define js_TracingAPI_h

#include "jsalloc.h"

#include "js/HashTable.h"
#include "js/HeapAPI.h"
#include "js/TraceKind.h"

class JS_PUBLIC_API(JSTracer);

namespace JS {
class JS_PUBLIC_API(CallbackTracer);
template <typename T> class Heap;
template <typename T> class TenuredHeap;

/** Returns a static string equivalent of |kind|. */
JS_FRIEND_API(const char*)
GCTraceKindToAscii(JS::TraceKind kind);

} // namespace JS

enum WeakMapTraceKind {
    /**
     * Do not trace into weak map keys or values during traversal. Users must
     * handle weak maps manually.
     */
    DoNotTraceWeakMaps,

    /**
     * Do true ephemeron marking with a weak key lookup marking phase. This is
     * the default for GCMarker.
     */
    ExpandWeakMaps,

    /**
     * Trace through to all values, irrespective of whether the keys are live
     * or not. Used for non-marking tracers.
     */
    TraceWeakMapValues,

    /**
     * Trace through to all keys and values, irrespective of whether the keys
     * are live or not. Used for non-marking tracers.
     */
    TraceWeakMapKeysValues
};

class JS_PUBLIC_API(JSTracer)
{
  public:
    // Return the runtime set on the tracer.
    JSRuntime* runtime() const { return runtime_; }

    // Return the weak map tracing behavior currently set on this tracer.
    WeakMapTraceKind weakMapAction() const { return weakMapAction_; }

    enum class TracerKindTag {
        // Marking path: a tracer used only for marking liveness of cells, not
        // for moving them. The kind will transition to WeakMarking after
        // everything reachable by regular edges has been marked.
        Marking,

        // Same as Marking, except we have now moved on to the "weak marking
        // phase", in which every marked obj/script is immediately looked up to
        // see if it is a weak map key (and therefore might require marking its
        // weak map value).
        WeakMarking,

        // A tracer that traverses the graph for the purposes of moving objects
        // from the nursery to the tenured area.
        Tenuring,

        // General-purpose traversal that invokes a callback on each cell.
        // Traversing children is the responsibility of the callback.
        Callback
    };
    bool isMarkingTracer() const { return tag_ == TracerKindTag::Marking || tag_ == TracerKindTag::WeakMarking; }
    bool isWeakMarkingTracer() const { return tag_ == TracerKindTag::WeakMarking; }
    bool isTenuringTracer() const { return tag_ == TracerKindTag::Tenuring; }
    bool isCallbackTracer() const { return tag_ == TracerKindTag::Callback; }
    inline JS::CallbackTracer* asCallbackTracer();
#ifdef DEBUG
    bool checkEdges() { return checkEdges_; }
#endif

  protected:
    JSTracer(JSRuntime* rt, TracerKindTag tag,
             WeakMapTraceKind weakTraceKind = TraceWeakMapValues)
      : runtime_(rt)
      , weakMapAction_(weakTraceKind)
#ifdef DEBUG
      , checkEdges_(true)
#endif
      , tag_(tag)
    {}

#ifdef DEBUG
    // Set whether to check edges are valid in debug builds.
    void setCheckEdges(bool check) {
        checkEdges_ = check;
    }
#endif

  private:
    JSRuntime* runtime_;
    WeakMapTraceKind weakMapAction_;
#ifdef DEBUG
    bool checkEdges_;
#endif

  protected:
    TracerKindTag tag_;
};

namespace JS {

class AutoTracingName;
class AutoTracingIndex;
class AutoTracingCallback;

class JS_PUBLIC_API(CallbackTracer) : public JSTracer
{
  public:
    CallbackTracer(JSRuntime* rt, WeakMapTraceKind weakTraceKind = TraceWeakMapValues)
      : JSTracer(rt, JSTracer::TracerKindTag::Callback, weakTraceKind),
        contextName_(nullptr), contextIndex_(InvalidIndex), contextFunctor_(nullptr)
    {}
    CallbackTracer(JSContext* cx, WeakMapTraceKind weakTraceKind = TraceWeakMapValues);

    // Override these methods to receive notification when an edge is visited
    // with the type contained in the callback. The default implementation
    // dispatches to the fully-generic onChild implementation, so for cases that
    // do not care about boxing overhead and do not need the actual edges,
    // just override the generic onChild.
    virtual void onObjectEdge(JSObject** objp) { onChild(JS::GCCellPtr(*objp)); }
    virtual void onStringEdge(JSString** strp) { onChild(JS::GCCellPtr(*strp)); }
    virtual void onSymbolEdge(JS::Symbol** symp) { onChild(JS::GCCellPtr(*symp)); }
    virtual void onScriptEdge(JSScript** scriptp) { onChild(JS::GCCellPtr(*scriptp)); }
    virtual void onShapeEdge(js::Shape** shapep) {
        onChild(JS::GCCellPtr(*shapep, JS::TraceKind::Shape));
    }
    virtual void onObjectGroupEdge(js::ObjectGroup** groupp) {
        onChild(JS::GCCellPtr(*groupp, JS::TraceKind::ObjectGroup));
    }
    virtual void onBaseShapeEdge(js::BaseShape** basep) {
        onChild(JS::GCCellPtr(*basep, JS::TraceKind::BaseShape));
    }
    virtual void onJitCodeEdge(js::jit::JitCode** codep) {
        onChild(JS::GCCellPtr(*codep, JS::TraceKind::JitCode));
    }
    virtual void onLazyScriptEdge(js::LazyScript** lazyp) {
        onChild(JS::GCCellPtr(*lazyp, JS::TraceKind::LazyScript));
    }
    virtual void onScopeEdge(js::Scope** scopep) {
        onChild(JS::GCCellPtr(*scopep, JS::TraceKind::Scope));
    }

    // Override this method to receive notification when a node in the GC
    // heap graph is visited.
    virtual void onChild(const JS::GCCellPtr& thing) = 0;

    // Access to the tracing context:
    // When tracing with a JS::CallbackTracer, we invoke the callback with the
    // edge location and the type of target. This is useful for operating on
    // the edge in the abstract or on the target thing, satisfying most common
    // use cases.  However, some tracers need additional detail about the
    // specific edge that is being traced in order to be useful. Unfortunately,
    // the raw pointer to the edge that we provide is not enough information to
    // infer much of anything useful about that edge.
    //
    // In order to better support use cases that care in particular about edges
    // -- as opposed to the target thing -- tracing implementations are
    // responsible for providing extra context information about each edge they
    // trace, as it is traced. This contains, at a minimum, an edge name and,
    // when tracing an array, the index. Further specialization can be achived
    // (with some complexity), by associating a functor with the tracer so
    // that, when requested, the user can generate totally custom edge
    // descriptions.

    // Returns the current edge's name. It is only valid to call this when
    // inside the trace callback, however, the edge name will always be set.
    const char* contextName() const { MOZ_ASSERT(contextName_); return contextName_; }

    // Returns the current edge's index, if marked as part of an array of edges.
    // This must be called only inside the trace callback. When not tracing an
    // array, the value will be InvalidIndex.
    const static size_t InvalidIndex = size_t(-1);
    size_t contextIndex() const { return contextIndex_; }

    // Build a description of this edge in the heap graph. This call may invoke
    // the context functor, if set, which may inspect arbitrary areas of the
    // heap. On the other hand, the description provided by this method may be
    // substantially more accurate and useful than those provided by only the
    // contextName and contextIndex.
    void getTracingEdgeName(char* buffer, size_t bufferSize);

    // The trace implementation may associate a callback with one or more edges
    // using AutoTracingDetails. This functor is called by getTracingEdgeName
    // and is responsible for providing a textual representation of the
    // currently being traced edge. The callback has access to the full heap,
    // including the currently set tracing context.
    class ContextFunctor {
      public:
        virtual void operator()(CallbackTracer* trc, char* buf, size_t bufsize) = 0;
    };

#ifdef DEBUG
    enum class TracerKind { DoNotCare, Moving, GrayBuffering, VerifyTraceProtoAndIface };
    virtual TracerKind getTracerKind() const { return TracerKind::DoNotCare; }
#endif

    // In C++, overriding a method hides all methods in the base class with
    // that name, not just methods with that signature. Thus, the typed edge
    // methods have to have distinct names to allow us to override them
    // individually, which is freqently useful if, for example, we only want to
    // process only one type of edge.
    void dispatchToOnEdge(JSObject** objp) { onObjectEdge(objp); }
    void dispatchToOnEdge(JSString** strp) { onStringEdge(strp); }
    void dispatchToOnEdge(JS::Symbol** symp) { onSymbolEdge(symp); }
    void dispatchToOnEdge(JSScript** scriptp) { onScriptEdge(scriptp); }
    void dispatchToOnEdge(js::Shape** shapep) { onShapeEdge(shapep); }
    void dispatchToOnEdge(js::ObjectGroup** groupp) { onObjectGroupEdge(groupp); }
    void dispatchToOnEdge(js::BaseShape** basep) { onBaseShapeEdge(basep); }
    void dispatchToOnEdge(js::jit::JitCode** codep) { onJitCodeEdge(codep); }
    void dispatchToOnEdge(js::LazyScript** lazyp) { onLazyScriptEdge(lazyp); }
    void dispatchToOnEdge(js::Scope** scopep) { onScopeEdge(scopep); }

  private:
    friend class AutoTracingName;
    const char* contextName_;

    friend class AutoTracingIndex;
    size_t contextIndex_;

    friend class AutoTracingDetails;
    ContextFunctor* contextFunctor_;
};

// Set the name portion of the tracer's context for the current edge.
class MOZ_RAII AutoTracingName
{
    CallbackTracer* trc_;
    const char* prior_;

  public:
    AutoTracingName(CallbackTracer* trc, const char* name) : trc_(trc), prior_(trc->contextName_) {
        MOZ_ASSERT(name);
        trc->contextName_ = name;
    }
    ~AutoTracingName() {
        MOZ_ASSERT(trc_->contextName_);
        trc_->contextName_ = prior_;
    }
};

// Set the index portion of the tracer's context for the current range.
class MOZ_RAII AutoTracingIndex
{
    CallbackTracer* trc_;

  public:
    explicit AutoTracingIndex(JSTracer* trc, size_t initial = 0) : trc_(nullptr) {
        if (trc->isCallbackTracer()) {
            trc_ = trc->asCallbackTracer();
            MOZ_ASSERT(trc_->contextIndex_ == CallbackTracer::InvalidIndex);
            trc_->contextIndex_ = initial;
        }
    }
    ~AutoTracingIndex() {
        if (trc_) {
            MOZ_ASSERT(trc_->contextIndex_ != CallbackTracer::InvalidIndex);
            trc_->contextIndex_ = CallbackTracer::InvalidIndex;
        }
    }

    void operator++() {
        if (trc_) {
            MOZ_ASSERT(trc_->contextIndex_ != CallbackTracer::InvalidIndex);
            ++trc_->contextIndex_;
        }
    }
};

// Set a context callback for the trace callback to use, if it needs a detailed
// edge description.
class MOZ_RAII AutoTracingDetails
{
    CallbackTracer* trc_;

  public:
    AutoTracingDetails(JSTracer* trc, CallbackTracer::ContextFunctor& func) : trc_(nullptr) {
        if (trc->isCallbackTracer()) {
            trc_ = trc->asCallbackTracer();
            MOZ_ASSERT(trc_->contextFunctor_ == nullptr);
            trc_->contextFunctor_ = &func;
        }
    }
    ~AutoTracingDetails() {
        if (trc_) {
            MOZ_ASSERT(trc_->contextFunctor_);
            trc_->contextFunctor_ = nullptr;
        }
    }
};

} // namespace JS

JS::CallbackTracer*
JSTracer::asCallbackTracer()
{
    MOZ_ASSERT(isCallbackTracer());
    return static_cast<JS::CallbackTracer*>(this);
}

namespace JS {

// The JS::TraceEdge family of functions traces the given GC thing reference.
// This performs the tracing action configured on the given JSTracer: typically
// calling the JSTracer::callback or marking the thing as live.
//
// The argument to JS::TraceEdge is an in-out param: when the function returns,
// the garbage collector might have moved the GC thing. In this case, the
// reference passed to JS::TraceEdge will be updated to the thing's new
// location. Callers of this method are responsible for updating any state that
// is dependent on the object's address. For example, if the object's address
// is used as a key in a hashtable, then the object must be removed and
// re-inserted with the correct hash.
//
// Note that while |edgep| must never be null, it is fine for |*edgep| to be
// nullptr.
template <typename T>
extern JS_PUBLIC_API(void)
TraceEdge(JSTracer* trc, JS::Heap<T>* edgep, const char* name);

extern JS_PUBLIC_API(void)
TraceEdge(JSTracer* trc, JS::TenuredHeap<JSObject*>* edgep, const char* name);

// Edges that are always traced as part of root marking do not require
// incremental barriers. This function allows for marking non-barriered
// pointers, but asserts that this happens during root marking.
//
// Note that while |edgep| must never be null, it is fine for |*edgep| to be
// nullptr.
template <typename T>
extern JS_PUBLIC_API(void)
UnsafeTraceRoot(JSTracer* trc, T* edgep, const char* name);

extern JS_PUBLIC_API(void)
TraceChildren(JSTracer* trc, GCCellPtr thing);

using ZoneSet = js::HashSet<Zone*, js::DefaultHasher<Zone*>, js::SystemAllocPolicy>;
using CompartmentSet = js::HashSet<JSCompartment*, js::DefaultHasher<JSCompartment*>,
                                   js::SystemAllocPolicy>;

/**
 * Trace every value within |compartments| that is wrapped by a
 * cross-compartment wrapper from a compartment that is not an element of
 * |compartments|.
 */
extern JS_PUBLIC_API(void)
TraceIncomingCCWs(JSTracer* trc, const JS::CompartmentSet& compartments);

} // namespace JS

extern JS_PUBLIC_API(void)
JS_GetTraceThingInfo(char* buf, size_t bufsize, JSTracer* trc,
                     void* thing, JS::TraceKind kind, bool includeDetails);

namespace js {

// Trace an edge that is not a GC root and is not wrapped in a barriered
// wrapper for some reason.
//
// This method does not check if |*edgep| is non-null before tracing through
// it, so callers must check any nullable pointer before calling this method.
template <typename T>
extern JS_PUBLIC_API(void)
UnsafeTraceManuallyBarrieredEdge(JSTracer* trc, T* edgep, const char* name);

namespace gc {

// Return true if the given edge is not live and is about to be swept.
template <typename T>
extern JS_PUBLIC_API(bool)
EdgeNeedsSweep(JS::Heap<T>* edgep);

// Not part of the public API, but declared here so we can use it in GCPolicy
// which is.
template <typename T>
bool
IsAboutToBeFinalizedUnbarriered(T* thingp);

} // namespace gc
} // namespace js

#endif /* js_TracingAPI_h */